An earthquake of magnitude 6.5 (M6.5) struck in the Gulf of California around 50 miles from the Mexican coast on 19 October, at a depth of just 5 miles. At the time of writing there were no reports of any damage or injuries, and no tsunami warning was issued, although reports from the United States Geological Survey indicated that residents as far away as Los Angeles felt the quake.
Tectonic Setting for the 19 October Gulf of California Earthquake
The tectonic setting of the whole of the Gulf of California is interesting; the plate margin between the Pacific and North American plates is the southward extension of the notorious San Andreas fault zone, which is a series of major, mostly strike-slip, faults along which the two plates slide past one another. At the extreme south of this zone, however, the nature of the margin changes and becomes a constructive boundary (marked onshore in California’s Salton Sea area by geothermal activity).
The plate boundary marks the axis of the Gulf but is not a single spreading ridge: Instead it consists of spreading centres which alternate with offset faults, between them accommodating movement in different directions. The epicentre of the earthquake, which is located just to the west of the main boundary, suggests that the mechanism for the tremor was movement along one of these offset transform faults.
Gulf of California Earthquakes
The earthquake of October 19 is by no means unusual within the Gulf. Yeats refers to several large strike-slip earthquakes along the offset transform faults, including one of M6.9 in August 2009, which was accompanied by several significant aftershocks: the area also experienced earthquakes of M6.7 (2010), M6.6 (2006) and M6.5 (1975) making the most recent a noteworthy event in the area.
In the earthquake annals of Mexico, however, it can be regarded as small. Further south, beyond the mouth of the Gulf, the nature of the plate margin changes again, to a convergent boundary, where the subduction of the Cocos plate beneath central America is the source of frequent large, often damaging earthquakes.
All of Mexico’s largest tremors, and the most deadly, are associated with this destructive boundary: the country’s largest, an M8.1 in 1932, is many time s large than anything recorded to date in the Gulf.
Baja California: a Microcontinent in the Making?
The tectonic setting of the October 19 earthquake is particularly interesting as the divergent nature of the boundary, with new ocean crust being generated, is effectively widening the Gulf of California by around 50mm per year (USGS). Over millions of years this could separate the Baja California peninsula from the rest of North America creating a microcontinent – part of the constant process of cycling and recycling of the continents resulting from tectonic movement.
USGS. M6.5 – 75km south of Etchoropo, Mexico. (2013). Accessed 20 October 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.